This invention relates to a.c. motor start and run systems and more particularly to such systems that employ thermistors for assisting start. In such systems, the motor has a main winding, and a start or auxillary winding. During start both windings are energized. During normal run conditions the main winding is always energized whereas the start winding for certain motor designs is deenergized and for other cases the running current in the start winding is supplied at a reduced value. Potential relays, current relays, and centrifugal switches have been traditionally used to provide this function. These devices tend to be expensive and unreliable.
It is known PTCR use a thermistor such as a positive temperature coefficient resistor (PTCR) as a time delay element in a motor control system for automatically determining the start period duration avoiding the shortcomings of the aforementioned traditional devices. The PTCRs of interest herein are of the type having an anomaly temperature above which the electrical impedance of the PCTR is several orders of magnitude greater than it is for temperatures below the anomaly temperature.
A second function that a motor control system desirably performs is that of preventing restart for a period after deenergization to allow, for instance, a time for the motor driven compressor system to equalize pressure, or to allow time for an overheated motor to cool to a safe temperature. Electromechanical timers have been used to provide this function as have temperature responsive elements or switches that are thermally coupled to the motor.
The conventional system using a single PTCR in series with a motor start winding to accomplish the start function, has the shortcoming of a non-definitive restart function such that a premature attempt to restart the motor has the effect of providing an abnormally short start period that in turn may result in improper or sluggish starting. Furthermore, in many motor start systems it is desirable to keep both the start and main windings disconnected from the line during the restart delay period, which the single PTCR system cannot do without the addition of more components and additional circuit complexity.
It is therefore an object of this invention to provide a simple low-cost motor control system.
It is a further object of this invention to provide a motor control system capable of disconnecting both start and main motor windings from the power line during a predetermined non-restart period.
It is a further object of this invention to provide a motor control system having a definitive restart characteristic.
It is a further object of this invention to employ in a motor control system a package of PTCRs whose thermal characteristics determine the motor start period and the delay to restart period.